Thin film transistors (TFTs) are one example of field-effect transistors, and widely used especially in liquid crystal display devices, and organic EL display devices, etc.
Conventionally, silicon is popularly used as a material from which a channel layer (semiconductor layer) constituting the TFTs is formed. Recently, TFTs whose channel layer is made from an oxide semiconductor has been intensively studied.
As some examples of TFTs using an oxide semiconductor, Patent Literature 1 discloses a transparent thin-film field-effect transistor in which a thin film of homologous compound monocrystal InMO3(ZnO)m(M=In, Fe, Ga, or Al, m=an integer not less than 1 but not less than 50) is used as an active layer. Meanwhile, Patent Literature 2 discloses an image display device in which an amorphous oxide whose electron carrier concentration is less than 1018/cm3 is used as an active layer of a field-effect transistor.
Further, Patent Literature 3 discloses a thin film transistor in which an oxide semiconductor whose main constituent elements are In-M-Zn—O (M is at least one of Ga, Al, and Fe) is used as a channel layer, wherein the channel layer made from the oxide semiconductor is covered with a protection film.